1. Field of the Invention
The present invention relates to the paper transport devices that transport paper, and to document reading apparatuses and image forming apparatuses provided with such paper transport devices.
2. Description of the Related Art
Some document reading apparatuses read images on documents while transporting the documents using a paper transport device. Furthermore, some image forming apparatuses copy an image of a document read by a document reading apparatus onto recording paper by printing the image onto the recording paper while transporting the recording paper using a paper transport device.
If, in such a document reading apparatus, the document is transported at an angle relative to the correct paper transport direction, the document image will be read at that angle. A document being transported at an angle relative to the correct paper transport direction will also be referred to as “angled travel” or “traveling at an angle” hereinafter.
If a recording paper travels at an angle, the image forming apparatus will record the image onto the recording paper at a corresponding angle. Accordingly, a resist roller is provided in the paper transport path of paper transport devices that transport documents, recording paper, and so on. By controlling the rotational speed of the resist roller, the leading edge of the paper butts against the resist roller and stops, which causes the paper to flex. The leading edge of the paper is aligned parallel with the resist roller due to the repulsive force of the paper. After the angled travel of the paper has been corrected in this manner, the paper is transported by the resist roller. The resist roller begins transporting the paper in tandem with the timing at which the image of the document is read, the timing at which the image is transferred onto recording paper, or the like, after the resist roller has been temporarily stopped.
JP-2001-39584A (Patent Document 1) discloses detecting an angled travel amount of paper using a pre-resist sensor and adjusting the operating timing of the resist roller, or in other words, the timing at which the resist roller begins transporting the paper, based on the detected angled travel amount.
Incidentally, in recent years, there have been significant increases in the printing speeds of image forming apparatuses, and the speeds at which documents, recording paper, and so on are transported have also increased as a result. However, when the speed at which paper is transported is increased, there is a problem in that angled travel of paper cannot be completely corrected through a single correction performed by the resist roller.
Therefore, for example, first and second resist rollers have been disposed at a distance from each other in the paper transport path on the upstream and downstream sides in the paper transport direction, and angled travel of the paper has been corrected favorably by first correcting the angled travel of the paper using the first resist roller and then once again correcting the angled travel of the paper using the second resist roller.
However, in the case where the first and second resist rollers are provided, the leading edge of the paper butts against and stops at both the first and second resist rollers. This causes the transport speed of the paper to decrease at both the first and second resist rollers. This has therefore interfered with increases in the transport speed of the paper.
Even with the technique disclosed in Patent Document 1, the transport speed of the paper decreases. Patent Document 1 discloses not stopping the resist roller when the paper experiences a low amount of angled travel. However, in this case, the operations described earlier, in which the resist roller first stops temporarily and then begins to transport the paper in accordance with the timing at which the image of the document is read, the timing at which the image is transferred onto recording paper, or the like, will not be carried out. In other words, there is a problem in that one of the functions of the resist roller is impaired.